In order for a solar cell to be able to generate electric power by light in a visible light region and a near infrared region, a glass plate for the solar cell (such as a cover glass or a glass substrate for a thin-film solar cell) is required to have sufficiently high visible light transmittance (hereinafter referred to as Tv) and solar transmittance (hereinafter referred to as Te).
Further, in solar thermal power generation wherein sunlight is collected and used as a heat source to carry out power generation, a glass plate for a light collecting mirror is required to have sufficiently high Tv and Te in order to minimize a loss of sunlight (particularly light in a near infrared region) by the light collecting mirror. This glass plate can be used as a plane plate or curved plate.
Therefore, as the glass plate for a solar cell or as the glass plate for a light collecting mirror, a high transmission glass plate (so-called white plate glass) having Tv and Te made high (e.g. Tv of at least 90% , and Te of at least 90%) by minimizing the content of a coloring component (particularly iron) is used (Patent Document 1).
Further, even a high transmission glass plate contains iron unavoidably included in its production. Therefore, in order to make Te sufficiently high in a high transmission glass plate, it becomes important to increase the proportion of trivalent iron having an absorption peak in the vicinity of a wavelength of 400 nm as much as possible and to reduce the proportion of divalent iron having an absorption peak in the wavelength region from 1,000 nm to 1,100 nm as much as possible, among the total iron contained in the high transmission glass plate (Patent Document 2).
However, such a high transmission glass plate has the following problems.
(i) In order to minimize the content of iron in the high transmission glass plate, it is necessary to minimize the content of iron in glass raw material. However, glass raw material having a minimized content of iron is expensive, and the raw material cost for the high transmission glass plate becomes high.
(ii) In order to minimize the proportion of divalent iron in the high transmission glass plate, it is necessary to lower the temperature of molten glass at the time of producing the high transmission glass plate by a float process or a downdraw method as compared with the case of producing a usual glass plate, and the productivity is poor.
Heretofore, in order to realize a high transmission glass plate, a method of adjusting the proportions of divalent iron and trivalent iron among total iron contained in the glass towards the trivalent iron side by adding a very small amount of an oxidizing agent, or a method of changing the matrix composition of the glass to move the position of absorption peak of trivalent iron, has been proposed.
For example, by incorporating from 0.025 to 0.20%, as represented by mass percentage, of cerium oxide as an oxidizing agent, the proportion of divalent iron having an absorption peak in the wavelength region from 1,000 nm to 1,100 nm is reduced among the total iron contained in the glass (Patent Document 3).
Further, it is disclosed that in a soda lime silica glass having a total amount of iron oxide calculated as ferric oxide being from 0.02 to 0.2 mass %, its matrix composition is made to comprise, as represented by mass percentage, from 69 to 75% of SiO2, from 0 to 3% of Al2O3, from 0 to 5% of B2O3, from 2 to 10% of CaO, less than 2% of MgO, from 9 to 17% of Na2O, from 0 to 8% of K2O, optionally fluorine, zinc oxide, zirconium oxide and less than 4% of barium oxide, wherein the total of oxides of alkaline earth metals is adjusted to be at most 10%, whereby the absorption band by divalent iron is moved towards the long wavelength side, and it is possible to produce window glass which is less colored than a soda lime silica glass having a usual matrix composition and which is excellent in infrared absorption (Patent Document 4).